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Spin–orbit torque switching of a ferromagnet with picosecond electrical pulses

Abstract

The development of approaches that can efficiently control the magnetization of magnetic materials is central to the creation of fast and low-power spintronic devices. Spin transfer torque can be used to electrically manipulate magnetic order in devices, but is typically limited to nanosecond timescales. Alternatively, spin–orbit torque can be employed, and switching with current pulses down to ~200 ps has been demonstrated. However, the upper limit to magnetization switching speed remains unestablished. Here, we show that photoconductive switches can be used to apply 6-ps-wide electrical pulses and deterministically switch the out-of-plane magnetization of a common thin cobalt film via spin–orbit torque. We probe the ultrafast magnetization dynamics due to spin–orbit torques with sub-picosecond resolution using the time-resolved magneto-optical Kerr effect (MOKE). We also estimate that the magnetization switching consumes less than 50 pJ in micrometre-sized devices.

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Fig. 1: Sample and switching behaviour via field and current.
Fig. 2: Set-up for the generation of picosecond electrical pulses and magneto-optical detection.
Fig. 3: MOKE micrographs of single 6-ps electrical pulses switching the magnetization via SOT.
Fig. 4: Time-resolved MOKE response due to a 3.7-ps electrical pulse and macrospin simulation.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

The code that reproduced the fits within this paper is available from the corresponding author upon reasonable request.

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Acknowledgements

We thank E. Vatoux, T. Ferté and both L. Badie and G. Lengaigne for the electromagnet construction, vibrating sample magnetometry measurements and sample preparation, respectively. We especially thank Y. Yang and C.-H. Lambert for their help in the first SOT switching trials, years ago. This work was supported by the Impact Project LUE-N4S, part of the French PIA project ‘Lorraine Université d’Excellence’, reference ANR-15IDEX-04-LUE, and the ‘FEDER-FSE Lorraine et Massif Vosges 2014–2020’, a European Union Programme. This work was also partly supported by the French RENATECH network. R.L.C. and J.B. acknowledge support from the National Science Foundation (NSF) through Cooperative Agreement Award EEC-1160504 for Solicitation NSF 11-537 (TANMS). A.P. and J.B. also acknowledge support from the NSF Center for Energy Efficient Electronics (E3S). Work by X.S. and R.B.W. was supported by the US Army Research Laboratory and the US Army Research Office under contract/grant no. W911NF-18-1-0364. J.B. also acknowledges support by ASCENT (one of the SRC/DARPA supported centres within the JUMP initiative). Preliminary experiments in this work were supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05-CH11231 within the Nonequilibrium Magnetic Materials Program (MSMAG).

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J.G. designed the experiments with input from R.B.W., J.B., S.P.-W. and J.H. A.L. grew the LT-GaAs substrates. M.H. optimized and grew the samples by sputtering. K.J. fabricated the devices. K.J., J.H. and J.G. performed the ultrafast SOT experiments and characterized the picosecond pulses. E.M. and A.Y.A.C. performed the anomalous Hall measurements and 100-µs SOT switching experiments under the supervision of J.C.R.-S. and S.P.-W. R.B.W. built the numerical model and performed the simulations together with J.G. X.S. and R.B.W. performed optical time-resolved experiments to determine the damping and anisotropy of the samples. J.G. analysed the experimental data with help from K.J., R.B.W., R.L.C., J.H. and S.-P.W. J.G. wrote the manuscript with input from all authors.

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Correspondence to Jon Gorchon.

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Extended data

Extended Data Fig. 1 Dependence of quasi-static critical current density on in-plane field.

The critical current density for SOT switching with 100 µs pulses is inversely proportional to the in-plane Hx field, as reported previously ref. 23.

Extended Data Fig. 2 The switched area as a function the current density normalized by the critical current density.

JC, critical current density. The reversal process is first happening where the current density is the highest, at mid-height, right in between the tips of the rounded gold electrodes. To fully switch the device a little more current density than JC is needed. We note that these experiments were performed on a different sample with a coplanar stripline design. Moreover, the magnetic section had a ~5 μm width, slightly wider than the main sample of the article (4 μm). In fact, in the main sample, these partial reversals were not clearly evidenced. We believe this could be due to the narrower section, or also due to the gold contacts being flatter, possibly resulting in a more homogeneous current distribution. We also note that in the experiments of Supplementary Fig. 2 the current pulse duration is unknown (not measured). We estimate it in between 3-10 ps, from experience with similar devices.

Extended Data Fig. 3 Spatial dependence of 3.7 ps-wide SOT induced magnetization dynamics.

The time-resolved MOKE dynamics are shown as a function of the vertical position within the magnetic region, along the red dashed line. The colour of the traces corresponds to the positions indicated by the corresponding coloured circles in the sample picture. Inset shows the peak of the dynamics (at 11 ps) as a function of the y position (across the sample width). The signal drops as we get close to the edges because the probe no longer fully overlaps the magnet (the probe width is about 1.5 µm (FWHM), and the sample width is 4 µm. The dynamics are extremely similar across the surface of the sample. Experiments along the length of the magnet (x direction) also showed no major differences.

Extended Data Fig. 4 Coercivity as a function of the number of single 6 ps pulses spaced every 200 µs.

In order to check for a heat-assisted magnetic recording-like scenario, we injected single pulses at the switching threshold current, under no in-plane field, and monitored the variation of the coercive field (the applied out-of-plane field leading to ~50% reversal or more) with the number of applied current pulses. In a heat-assisted magnetic recording scenario, a single current pulse should be enough to lower the coercivity significantly. However, we did not observe any switching when injecting a single pulse and applying an out-of-plane field as large as 93% of the switching field. In fact, we only observe a small decrease of ~30% in the coercivity when increasing the number of pulses by a factor of 105. We conclude that the dissipation by a single electrical pulse does not lead to a heat-assisted recording scenario.

Extended Data Fig. 5 Calibration of picosecond current arrival time.

Time-domain thermoreflectance (black circles) and polar MOKE response (red line). The Time-domain thermoreflectance allows us to set time-zero in our experiment. The electrons immediately respond to the heat pulse (negative peak at time-zero). The magnetic dynamics (red) equally start at the arrival of the pulse with no noticeable delay. Further work is needed to fully interpret the thermoreflectance response.

Supplementary information

Supplementary Information

Supplementary Figs. 1–5, Notes 1–3 and Table 1.

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Jhuria, K., Hohlfeld, J., Pattabi, A. et al. Spin–orbit torque switching of a ferromagnet with picosecond electrical pulses. Nat Electron 3, 680–686 (2020). https://doi.org/10.1038/s41928-020-00488-3

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